1. Field of the Invention
This invention generally relates to a semiconductor device. This invention specifically relates to a high-frequency semiconductor device or a high-frequency integrated circuit used in a millimeter wave band (an extreme high frequency band, EHF band) to a submillimeter wave band. This invention also relates to a transmission line device or a coupled line device for a high-frequency signal.
2. Description of the Prior Art
Some heterojunction compound semiconductor transistors have cutoff frequencies higher than 100 GHz. Generally, operation of an electric circuit is more affected by parasitic capacitances and inductances as the frequency of a signal handled by the circuit increases. In a microwave band to a millimeter wave band, such parasitic reactances tend to cause serious problems in operation of an electric circuit.
Monolithic microwave integrated circuits (MMIC's) are known as devices which can solve the problems caused by parasitic reactances. The MMIC's have a semiconductor substrate on which inductors, capacitors, resistors, transmission lines, and transistors are formed together by a semiconductor processing technology. The MMIC's tend to be expensive. Since components (parts) of an MMIC are formed together, it is generally difficult to check the performance of each of the components.